1. Field of the Invention
This invention relates to an image processing device for correcting gradation data of color input image data that has been obtained by reading a document image with use of a copier, facsimile machine, scanner or the like.
2. Description of the Related Art
There sometimes rises a problem that image data obtained by reading a document image by a copier or the like may contain unnecessary image data that is undesired to be reproduced such as undertone and backside image. Particularly, in the case of reading a document image formed on color paper or a document image formed on both sides of extremely thin paper, data concerning the undertone or the backside image may likely to be included in the read image data.
Unnecessary image data such as undertone data generally consists of lower gradation components. Monochromatic copiers that have been used heretofore enable to remove such an unnecessary image data by setting a certain gradation as an uppermost gradation of the unnecessary image data (e.g., undertone data) and cutting off gradation components lower than the uppermost gradation.
As far as monochromatic image reading is concerned, it is possible to reproduce an image with a continuous gradation by taking the following measures. Specifically, as a further step of the above image processing, after removing the lower gradation components, the gradation range of the remaining part of the input image data including medium and higher gradation components is redefined as a new effective gradation range (for example, ranging from 0 to 255 gradations), thus producing new lower gradation components. This arrangement makes it possible to prevent forming a conspicuous gradation difference (step) around a boundary region between the unnecessary image data to be removed and the remaining part of the input image data.
In the case of color copiers that have been recently popular in the market, however, the above measures is not effective for the following reason when image processing is attempted to remove unnecessary image data such as undertone data from a scanned original color image data.
A color image is composed by mixing a plurality of different color components such as red (R), green (G), and blue (B) or cyan (C), magenta (M), and yellow (Y). Although mixing all the color components may produce a color different from undertone, there sometimes is a case that a certain image region of an original color image coincidentally contains a color component identical to a color component of the undertone.
Let's say that the certain image region contains a specific color component (for instance, yellow color component) which is identical to the undertone generally in a lower gradation range. For example, there is a case that an original document is tanned and accordingly the undertone thereof contains a yellow color component, and the original document image contains gradation data of a yellow color component of skin color of a human, which is coincidentally the same as the yellow color component of the undertone.
In such a case, removing all the lower gradation components corresponding to the undertone may altogether remove the lower gradation components that are not originated from the undertone data. Consequently, a reproduced color image does not contain the lower gradation components that have constituted part of the original color image any more, and accordingly, the reproduced color image may look different from the original color image, thus degrading color image reproducibility.
Further, since the gradation of the undertone generally differs color component by component, the uppermost gradation thereof to be cut off differs from color component to component. Accordingly, in image processing of a color image composed of different color components, it is considered to implement a gradation redefining process with respect to each of the color components in which the remaining gradation range (uncut portion) for each color component is magnified to fit to the available gradation range at a different gradation magnification rate with an attempt to suppress a gradation difference in the boundary region. However, this gradation redefining process may cause an unbalanced distribution of color components, thereby resulting in poor color image reproduction.